In the production of oil, a well is drilled downwardly from the surface to or through geological zones believed to contain oil. Casing is then positioned in the well bore for reinforcement. The casing is usually cemented in place to prevent crossflow of subsurface fluids between the zones. One or more strings of tubing are then run into the casing, after which oil is lifted through the tubing or external to the tubing and through the casing. Packers are sometimes employed to separate sections of the well between the casing and tubing so that fluids from different zones can be lifted through separate tubing strings.
It has become common practice in the industry to connect side pocket mandrels within the tubing strings at intervals in the well. Such mandrels include main bores in alignment with the interior of the tubing, and offset chambers or side pockets adapted to receive side pocket devices such as gas lift valves or the like. Wire line tools are thus intended to be passed through the tubing, while gas lift valves need to be selectively installed or removed from particular mandrels. In the past, this has oftentimes been done with the aid of kickover tools.
Side pocket mandrels are particularly useful during gas lift production techniques. Air or natural gas is introduced into the well and allowed to flow through the gas lift valve to reduce the specific gravity of the oil and thus facilitate lifting the oil to the surface. Such techniques are commonly employed with deep wells or with extremely heavy and viscous oil, where production by means of downhole pumps alone would otherwise be difficult.
Several difficulties, however, have been experienced with side pocket mandrels appearing in the prior art.
Because well mandrels operate under conditions of high internal and external pressure, these devices must be of rugged construction capable of withstanding extreme pressure for long periods of time. Specialized techniques, expensive machinery and large capital outlays have been required to manufacture prior art mandrels with adequate pressure ratings. Prior art manufacturing techniques have resulted in mandrels of thicker and heavier construction which occupy more space in the casing and thus reduce the remaining flow space for passage of oil therethrough.
In the past, it has been common to construct a side pocket mandrel from a specially machined oval pipe section, such as shown in FIG. 7 of U.S. Pat. No. 3,741,299, issued to Ben D. Terral. Such construction methods have been unsatisfactory, however, because a separate piece of pipe must be expensively machined for each various size tubing. More specifically, the various sizes of mandrels utilized in the industry have made prior art side pocket mandrels expensive and complicated to manufacture. The size mandrel required depends upon the tubing and gas lift valve sizes suitable for a particular well. Tubing sizes generally vary over a wide range. At the same time, the sizes of conventional gas lift valves only vary over a relatively limited range. In the prior art, each size mandrel had to be specially constructed, even though the same size gas lift valve could be used with several different mandrels having different sizes of associated tubing. A separate piece of oval shaped tubing had to be expensively machined for each gas lift valve and mandrel size combination. Expensive forging dies and upsetting machinery were often required to manufacture prior art mandrels. Prior art methods of constructing side pocket mandrels did not permit interchangeability of side pockets as a means of reducing the overall production cost of mandrels.
Other problems exist with regard to the internal configurations of these mandrels. In particular, the side pocket must be configured to exclude or discriminate against wire line tools which are intended to pass through the mandrel rather than seat in the side pocket. This problem is especially present in slanted or directional wells where tool hangup can occur frequently when the side pocket is positioned on the low side of the tubing. Costly production delays can result from wire line tools becoming caught in the mandrels.
While prior art methods of manufacture have exhibited at least a degree of utility in producing side pocket mandrels, and prior art side pocket mandrels have exhibited a degree of utility, room for significant improvement remains. A need has existed for a new and improved side pocket mandrel which can be manufactured at less expense, and which is structured to better discriminate between tools intended to engage the side pocket and tools which are intended to remain in the main bore of the mandrel regardless of the mandrel orientation.
The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known side pocket mandrels and methods of manufacturing such mandrels. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that side pocket mandrels and methods of manufacturing such mandrels appearing in the prior art have not been altogether satisfactory.